1. Field of the Invention
This invention relates to a device for measuring the thickness of thin layers, and more particularly, to such a device with a light supporting arm having a measuring probe at one end, a damping device at the other end, and a bearing device for the supporting arm. This invention also relates to the measurement method associated with the device and to a circuit for controlling such a device.
2. Discussion of Relevant Art
In some sectors of industry, primarily the automotive industry, very large numbers of items, in the range from 1 to 10 million, are produced. For example, the end faces of nozzles and anchorings of injection valves are chromeplated. The thickness of the chrome layers must lie within predetermined limits in the range from 3 to 8 .mu.m. The surfaces to be measured are annular with a typical diameter of 4 mm and a width of 0.5 to 1.5 mm. Components of this kind serve to inject a very precise quantity of fuel in an electronically controlled manner into the engine compartment. This is carried out over an interval whose cycle time has to be predetermined by the timing system. The typical number of stress cycles of the components is about 100 million, corresponding to an engine driving performance of about 160,000 km. If the chrome layers of the components are too small, there is a risk of sticking, i.e. of insufficient fuel passing into the engine compartment. Conversely, if the chrome layer is too thick, there is a risk that the valve will not close and that excessive quantities of fuel will be injected. In this case, there is even a risk of the engine being set on fire, a situation which generally entails manufacturer's liability.
For these reasons, it is necessary to measure these components not merely by random sampling, but also to check that each individual component is kept within the prescribed tolerances. Consequently, the measuring system used for this purpose must have a high resistance to loading, in order to be able continuously to measure the highly sensitive components, which are vital for correct functioning, at an acceptable cost.
Achieving a high service life of the measuring system is difficult in particular when measuring very hard and square-edged crystalline chrome layers.
A further difficulty when measuring thin layers is that there is always a risk when using tactile measuring methods that the layer to be measured may be deformed or altered in some other way during the measurement, resulting in incorrect measurements.